Smoke detectors, which have been commercially available since circa 1969, have proven to have a huge advantage in promising the safety of people if a fire breaks out in a house or building. But how do these detectors actually go about detecting smoke?
It turns out that smoke detectors use a couple of different techniques for recognizing danger. The first is the photoelectric method. In this system a beam of light is produced by the detector. When smoke enters the detector it interferes with the light beam, triggering the alarm. This type of detector works best with slow-moving fires that produce a lot of smoke.
The other (more complex) technique for detecting fires is ionization. Ionizing sensors rely on the element americium-241, a radioactive isotope of americium. This material is stored in a chamber between two metal plates, one of which is negatively charged while the other is positively charged. Alpha particles are emitted from the americium and are free to interact with the air inside the chamber between the two plates. The positively charged ions are attracted towards the negatively charged plate while negative charges head towards the positively charged plate. This flow of ions creates a steady current within the chamber.
When smoke from a fire enters this chamber, it binds with the charged particles and returns them to a neutral state. This upsets the current, and if the detected change is great enough, the alarm will sound. A smoke detector that works on an ionization sensor is quicker than a photoelectric sensor at detecting a fast-paced fire that produces little smoke.